It is considered by some to be, and should have been, a classic example of adaptive radiation. At the end of the Triassic the majority of pseudosuchians go extinct, removing the biggest competitors of the dinosaurs and leaving the door open for an evolutionary explosion of the dinosaurs. Interestingly, however, this is not what happened according to a new study by Steve Brusatte and colleagues (Brusatte et al., 2008b) who found, in the continuation of their research comparing morphospace disparity between ornithidirans and pseudosuchians (Brusatte et al., 2008a), that dinosaur disparity remained relatively unchanged through the Triassic/Jurassic boundary. It would be expected that once the extinction of the pseudosuchians freed up a large amount of morphospace, the dinosaur record (with whom the pseudosuchians occupied a lot of the same niches and had similar body plans) would show a strong response, yet the dinosaurs show only a "slight non-significant increase" (Brusatte et al., 2008b). Thus, these authors argue, "different aspects of dinosaur radiation (diversity, disparity, and abundance) were decoupled, and the overall macroevolutionary pattern of the first 50 Myr of dinosaur evolution is more complex than often considered (Brusatte et al., 2008b).

Adam Yates had discussed this (and his hypothesis) a few weeks ago at Dracovenator and I had provided some follow-up discussion here. Nonetheless, despite the timing of the extinction it is apparent that not to much is going on for the dinosaurs immediately after the TR/J extinction (which by the way took out the non-dinosaurian dinosauromorphs). Sure to the record of coelophysoids and sauropodomorphs you add a few large theropods such as Dilophosaurus and you see the first good records of heterodontosaurids and the earliest thyreophorans (including the first ornithischians and sauropodomorphs in N. America), but you do not see a true explosion of dinosaur diversity until you get to the Late Jurassic. How much of this is a sampling and/or preservation problem is unclear, but simply look at the Weishampel et al. (2004) chapter on dinosaur distribution in the 2nd edition of The Dinosauria and compare the faunal lists for these epochs. You really have to clean up the Late Triassic portion removing many of the Ornithischia references, indeterminate theropods (could be shuvosaurids), and all of the footprint evidence (no ornithischian or sauropodomorph tracks in N. America; the "theropod" tracks worldwide could be made by convergent dinosauriforms, and pseudosuchians), not to mention the really messed up stratigraphy for the Chinle and Dockum which caused some duplicate entries. Now compare the Late Triassic, Early Jurassic, and Middle Jurassic lists to the rest of the chapter. Surprised? I commend Brusatte et al. (2008a, 2008b) for setting the stage and providing a baseline framework for some much needed future research to address this enigma.

The new issue of the Journal of Vertebrate Paleontology [28(3)] contains a new paper by me (and coauthors Michelle Stocker and Randy Irmis) describing a new taxon of aetosaur. If you have never heard of aetosaurs they are heavily armored, probably omnivorous, crocodile-line archosaurs that are extremely common fossils in Late Triassic terrestrial deposits. Aetosaurs are characterized by their armor, which consists of row after row of rectangular armor plates (osteoderms). Interestingly the dorsal (upper) surface of these plates is ornamented and this armor pattern is diagnostic of taxa. Even more informative is the morphology of what are termed lateral plates, or plates protecting the flanks of the animal. All aetosaurs can roughly be divided into three groups (clades) based on lateral plate morphology (Parker, 2007).

This is the fourth (and last) paper in a series which reanalyzes the genus Desmatosuchus (see also Parker, 2005, 2007, 2008). In 2000 I was studying the collections at the University of Michigan Museum of Paleontology (UMMP) when I came across the partial skeleton of small aetosaur that had been assigned to the genus Desmatosuchus. This specimen immediately caught my attention because although it represented a Desmatosuchine, it was quite different from Desmatosuchus (the holotype was on display upstairs). However, this specimen was was a bit of an enigma because of its incompleteness, crushing (it was also overprepared), and the fact that it appeared to be a juvenile. I gave a presentation on this specimen back in 2001 at the Western Association of Vertebrate Paleontologist’s meeting where I concluded that it either represented a new taxon or was possibly a juvenile form of Longosuchus (another Desmatosuchine) (Parker, 2001). Despite moving on to other projects, this fossil was always in the back of my mind and in 2004 enlisted Michelle Stocker to work with me on the description. The specimen was distinct enough from both Desmatosuchus and Longosuchus to warrant erection as a new taxon; however, we were still bothered by the small size of the specimen because very little is known regarding ontogentic changes to aetosaur armor morphology. Then two things happened; 1) we discovered juvenile Typothorax (another aetosaur) material at Petrified Forest National Park that clearly showed that little if any change occurred through ontogeny; 2) Bill Mueller at Texas Tech University turned us on to a lateral plate from a larger individual that was clearly referable to the same taxon, and possessed the same characters as the UMMP specimen. Still, we needed to be sure so we recruited Randy Irmis to conduct histological work to determine an ontogentic stage for the material. Randy was able to conclude that although the specimen was not fully grown, it was not exactly a very young juvenile either. Thus we felt confident enough to complete the study and erect a new taxon.

This is the first published study that attempts ontogentic stage determination in an aetosaur using the histology of osteoderms, and we hope that this will become a very important tool for future studies. For those who are wondering, the name Sierritasuchus is from Sierrita de la Cruz Creek near where the specimen was found. The species name S. macalpini honors the late Archie MacAlpin who collected the specimen in 1939. MacAlpin was a student of Ermine Cowles Case who published much on the Late Triassic of Texas. MacAlpin was later a geology professor at the University of Notre Dame. Sierritasuchus is currently only known from two specimens from the Tecovas Formation (Dockum Group) of Texas. It differs from both Desmatosuchus and Longosuchus (the two best known desmatosuchines) by various characters of the osteoderms and vertebrae. The picture above shows the majority of the holotype material (UMMP V60817). The reconstruction for this post was generously completed by Jeff Martz.

Parker, W.G. 2005. A new species of the Late Triassic aetosaur Desmatosuchus (Archosauria: Pseudosuchia). Compte Rendus Palevol 4:327-340.

Parker, W.G. 2007. Reassessment of the aetosaur “Desmatosuchus” chamaensis with a reanalysis of the phylogeny of the Aetosauria (Archosauria: Pseudosuchia). Journal of Systematic Palaeontology 5:41-68.

Parker, W.G. 2008. Description of new material of the aetosaur Desmatosuchus spurensis (Archosauria: Suchia) from the Chinle Formation of Arizona and a revision of the genus Desmatosuchus. PaleoBios 28:1-40.

For the last couple of years one of the more intriguing hypotheses regarding the Chinle Formation is the possibility, based on proposed revisions of the Late Triassic time scale (Muttoni et al., 2004; Furin et al., 2006), that the entire unit was Norian-Rhaetian in age, rather than Carnian-Norian. The Carnian-Norian age was based on palynology (e.g., Litwin et al., 1991) as well as vertebrate biostratigraphy (e.g., Lucas and Hunt, 1993). In fact, a faunal and floral turnover near the middle of the Chinle Formation was hypothesized by many workers to possibly represent the Carnian-Norian boundary and was evidence for an end-Carnian terrestrial extinction. First noticed by Camp (1930) and Gregory (1957) this turnover was more fully documented by Long and Ballew (1985) who noted the presence of two distinct faunas differentiated by the phytosaurs “Rutiodon A” and “Rutiodon B”, as well as the aetosaurs Calyptosuchus and Typothorax. Lucas and Hunt (1993) subsequently named these the Adamanian and Revueltian land-vertebrate faunachrons.

Considered controversial, the hypothesis that most if not all of the Chinle is actually Norian received strong support by the recent announcement of a new 206Pb/238U age of 219.2 ± 0.7 Ma from the base of the Blue Mesa Member in New Mexico (Mundil and Irmis, 2008). This is important for several reasons, firstly, the Chinle Formation has very well documented vertebrate, invertebrate, plant and trace fossil assemblages and is arguably a keystone unit for study of the Late Triassic terrestrial record. Second, it is the basis for the Adamanian land-vertebrate faunachron which has been used to correlate the first appearance of dinosaurs globally (e.g., Heckert and Lucas 1999, 2000). The implications of this will be the focus of Randy Irmis’ presentation at the Society of Vertebrate Paleontology annual meeting next month and therefore will not be discussed further here. What I would like to discuss is the slight possibility that despite these new findings maybe not all of the lower Chinle is now Norian.

In the four corners are of the western United States (Arizona, Utah, Colorado, New Mexico), the Chinle Formation uncomformably overlies the Middle Triassic (Anisian) Moenkopi Formation and consists of seven members which from oldest to youngest are the Shinarump, Bluewater Creek, Blue Mesa, Sonsela, Petrified Forest, Owl Rock, and Rock Point (Note: another unit, the Mesa Redondo Member, is locally situated between the Shinarump and Blue Mesa members but is lithologically distinct from the Bluewater Creek and the relationships between these units are not completely understood). The date provided by Mundil and Irmis (2008) is near the Bluewater Creek/Blue Mesa contact. As the new hypothesized date for the Carnian-Norian boundary is now around 230 ma, this would still leave approximately 10 million years of Norian time below this contact and would presumably include the Shinarump and Bluewater Creek. This is important because instead of just having an uncomformity between the Moenkopi and Chinle that encompasses the Ladinian, this unconformity would now encompass the Ladinian-Carnian as well as a portion of the early Norian. Whereas this might possibly explain the lack of rhynchosaur material (believed to have died out at the end of the Carnian or in the early Norian) from the Chinle Formation, it would also suggest that the Chinle is not temporally equivalent to all of the Dockum Group (eastern New Mexico and Texas) or most of the Newark Supergroup (eastern U.S.) as previously supposed. I’m getting dangerously close to the contents of Randy’s SVP abstract here which is embargoed for the next month, so I will stop this line of thought, but what I want to look at in more detail is the possibility that the Shinarump may not be Norian.

The Shinarump member consists mainly of extrabasinal conglomerates and sandstones that fill paleovalleys carved into the underlying Moenkopi Formation. Once considered its own formation, the Shinarump is now considered to represent the basal member of the Chinle Formation. Unfortunately, the high energy environment that deposited the conglomerates and sands is not conducive to preserving fossils; however in some places mudstone facies do preserve material, most notably near Cameron Arizona. Ash (2005, 2006) has documented this flora and found that it is distinct from the rest of the Chinle Formation in possessing several forms, most notably a seed fern, that more closely resembles archaic forms from the Paleozoic. This suggests that the Shinarump, although still Late Triassic, may be much older than the rest of the Chinle. Unfortunately the vertebrates are not any help. Heckert et al. (2003) documented material that is purportedly from the Shinarump near Cameron, and found that the fauna contains metoposaur and phytosaur material typical of the rest of the Chinle. Furthermore, the flora does also contain forms found in the younger Chinle units including the pollen (Litwin et al., 1991; Ash, 2005). Finally in his excellent dissertation, Jeff Martz discusses in detail that Riggs et al. (1996), in an important but often overlooked paper, used detrital zircons to correlate the Shinarump (and the Santa Rosa Formation of the Dockum) with the marine Auld Lang Syne Group, which is early Norian in age (Martz, 2008). Thus, these authors (Riggs et al., 1996) had suggested a Norian age for the entire Chinle over a decade ago. Despite this, the idea that the Shinarump may still be Carnian is intriguing because it would suggest the presence of a sizeable unconformity (TR-4?) between that unit and the rest of the Chinle Formation.

One final note is that the faunal turnover mentioned at the beginning of this post is close to the base of the Sonsela Member based on detailed mapping and revised biostratigraphic work done by Jeff Martz and myself in Petrified Forest National Park. Thus this turnover (which may also correspond with a floral turnover) is in the early-middle Norian and does not represent an end-Carnian event. More on this later.

Gregory, J.T. 1957. Significance of fossil vertebrates for correlation of Late Triassic continental deposits of North America. Report of the 20th Session of the International Geological Congress 1956, Section II:7-25.

Long, R. A. and K. L. Ballew. 1985. Aetosaur dermal armor from the Late Triassic of southwestern North America, with special reference to material from the Chinle Formation of Petrified Forest National Park. Museum of Northern Arizona Bulletin 54:45-68.

Lucas, S.G., and A.P. Hunt. 1993. Tetrapod biochronology of the Chinle Group (Upper Triassic), western United States. New Mexico Museum of Natural History and Science Bulletin 3:327-329.

The Society of Vertebrate Paleontology has posted a downloadable version of the abstract volume for the annual meeting in Cleveland at their website. The volume contains quite a few important abstracts on Late Triassic research; however, it is only downloadable by members and the SVP has noted that all abstracts are "embargoed until the day and time of their individual presentation". Of course I'm taking this to mean that online discussion of the contents of various abstracts is a no-no. This is of interest because as far as I am aware the SVP has never before made such a request. I am assuming that I will be receiving my print copy of the abstract volume within a few days and thus the abstracts will be in published form before the meeting (yet still embargoed?). Unfortunately no clarification was given by the SVP regarding this. Of course I will honor this request although my next planned post (within a few days) will discuss the implications of an abstract from an earlier meeting this year; however, some of the information duplicates an abstract by the same authors for the upcoming SVP meeting.

Very recently there has been a resurgence of interest in the early appearance and diversification of the Dinosauria mainly due to the recognition that there exists strong convergence between early dinosaurs and pseudosuchian archosaurs such as Revueltosaurus and Shuvosaurus, and that dinosaur precursors such as Dromomeron and Silesaurus not only survived into the Late Triassic but also coexisted with the dinosaurs for millions of years (Dzik, 2003; Ezcurra, 2006; Irmis et al. 2007b; Nesbitt et al., 2007; Parker et al., 2005; Nesbitt and Norell, 2006). This has been accompanied by studies demonstrating that in some faunas (especially those of North America) dinosaurs were neither dominant or diverse, and that in fact there is no unambiguous evidence of Triassic ornithischians or sauropodomorphs in North America, and that the global record of Triassic ornithischians is extremely poor (Irmis et al., 2007a; Nesbitt et al., 2007). These and other studies have also demonstrated that Late Triassic pseudosuchians were extremely diverse and that their occurrence together with ornithodirans in most Late Triassic assemblages demonstrates that they were filling similar ecological roles. Thus, one of the biggest mysteries is why the majority of pseudosuchian lineages die out at the end of the Triassic, while the more conservative dinosaurs go on to have great success for the next 140 million years.

Today in the new issue of Science, Brusatte et al. provide the results of a multifaceted study addressing this question. They provide a new phylogenetic analysis of the Archosauria (supplementary materials) and compare evolutionary rates and morphological disparity between pseudosuchians and ornithodirans. Interestingly they found that the dinosaurs had lower disparity and represented a lesser amount of morphospace occupation compared to the pseudosuchians. Furthermore rates of character evolution between the two groups were indistinguishable. Previous hypotheses that the dinosaurs were more successful due to physiological superiority and were “preordained for success” are discounted (as was also argued by Irmis et al., 2007b). Instead Brusatte et al., suggest that the “dinosaurs were the beneficiaries of two mass extinction events – and some good luck”.

I admit that I am not surprised at all by their findings, but am probably biased because this trend is readily apparent in North America (where I work) where with the exception of the Hayden and Coelophysis Quarries at Ghost Ranch New Mexico (and trackways in the youngest Triassic units) there is a marked paucity of Triassic dinosaur fossils and an abundance of diverse pseudosuchians. I am a bit flummoxed over the basal positioning of Revueltosaurus in their phylogeny, but this is based on an incomplete coding which I have not thoroughly reviewed.

Overall I find the paper to be a useful contribution in the attempt to discern why such a wonderful diversity of crocodile-line archosaurs lineages was extinguished at the end Triassic. Their data helps quantify some of the trends seen by other workers, especially that the competition model is most likely untenable. However, disproving the competition scenario does not necessarily support the "lucky break" hypothesis. Furthermore, I have not seen strong evidence for a Carnian-Norian terrestrial extinction in the fossil record, a claim that is even more weakened by the recent announcement of a Rhaetian dicynodont, which supports known Norian dicynodonts in Arizona and rhynchosaurs in Brazil and Argentina. Recent published and unpublished studies revising the Late Triassic timescale demonstrate that much of the hypothesized Carnian terrestrial strata worldwide is probably actually Norian, thus at best there are very few Carnian age terrestrial assemblages (e.g., Muttoni et al., 2004; Furin et al., 2004). There is still much work to be done on this mystery and I for one am not quite ready yet to simply attribute it a “lucky break”; however if this is the case then I truly rue what would appear to be a cruel twist of fate, and can only wonder what might have come to pass if the pendulum had swung the other way.

Dzik, J. A beaked herbivorous archosaurs with dinosaur affinities from the early Late Triassic of Poland. Journal of Vertebrate Paleontology 23:556-574.

Ezcurra, M.D. 2007. A review of the systematic position of the dinosauriform archosaur Eucoelophysis baldwini Sullivan & Lucas, 1999 from the Upper Triassic of New Mexico, USA. Geodiversitas 28:649-684.

Nesbitt, S.J, and M.A. Norell. 2006. Extreme convergence in the body plans of anearly suchian (Archosauria) and ornithomimid dinosaurs (Theropoda). Proceedings of the Royal Society of London Series B 273: 1045–1048.

Nesbitt, S.J., Irmis, R.B., and W.G. Parker, 2007. A critical reevaluation of the Late Triassic dinosaur taxa of North America. Journal of Systematic Palaeontology 5:209-243.

Parker, W.G., Irmis, R.B., Nesbitt, S.N., Martz, J. W., and L. S. Browne, 2005. The pseudosuchian Revueltosaurus callenderi and its implications for the diversity of early ornithischian dinosaurs. Proceedings of the Royal Society London B 272:963-969.

Adam Yates most recent post over at Dracovenator and a new abstract by Zeigler and Geissman has got me thinking more about faunal transitions between the Late Triassic and Middle Jurassic. As I stated in an earlier post, Chinle Formation faunal composition remains relatively consistent from the oldest to youngest localities and it is not until you get into the uppermost units of the formation and higher that you start to see some changes. Lucas and Tanner (2007) provides a good documentation of faunal change in the western U.S.A. through this interval and demonstrates that the lowermost Dinosaur Canyon Member (Moenave Formation) and the basal portion of the Wingate Sandstone (both units previously argued to be Jurassic in age and in the Glen Canyon Group) are most likely latest Triassic in age. This is based several lines of evidence including magnetostratigraphy, lithostratigraphic correlation, and biostratigraphy (the presence of phytosaur body fossils and pseudosuchian trace fossils). The upper Moenave, upper Wingate, and the Kayenta Formation lack these fossils. In addition, Lucas and Tanner (2007) place the youngest known Chinle Formation fossil assemblage (the Ghost Ranch Coelophysis Quarry) in the Rock Point Member, which they consider to be laterally equivalent to the base of the Wingate and the lower Dinosaur Canyon Member. They also consider this assemblage to be latest Norian in age based on palynology and the presence of the aetosaur Aetosaurus.

Zeigler and Geissman (2008) argue that based on magnetostratigraphy that the Ghost Ranch Coelophysis Quarry is not in the Rock Point and that it may be even younger than previously supposed. As I have noted previously, Zeigler (2008) correlates the site (using magnetostratigraphy) with the lower Moenave and now Zeigler and Geissman (2008) suggest that the uppermost Chinle Formation is at least Rhaetian and may even be Hettangian in age! This would extend the range of phytosaurs and other non-crocodylomorph pseudosuchians into the Early Jurassic. Thus there would be no terrestrial Triassic/Jurassic extinction, at least not in western North America.

Furthermore, Adam Yates recent post suggests that there may have been an end Early Jurassic extinction that spelled the end of coelophysoids and basal sauropodomorphs, followed by the rise of tetanurans and eusauropods in the Middle Jurassic. If Zeigler and Geissman and Yates are correct there would have been two major faunal turnovers in the very short period of time (approx. 30 million years) encompassing the Early Jurassic. In the earliest Jurassic we would see the disappearance of non-crocodylomorph pseudosuchians and the rise of a dinosaur dominated fauna, including the first basal sauropodomorphs in North America (which are not found in the Late Triassic of that continent*). Approximately 24 million years later we get the Early-Middle Jurassic turnover discussed by Yates and an explosion in dinosaurian diversity. Very interesting and the reason why research on the vertebrate fossil record of the lower Glen Canyon Group in becoming very important and needs to be expanded.

*Note: the only purported evidence of Late Triassic sauropodomorphs in North America are the ichnotaxa Tetrasauropus and Pseudotetrasauropus (e.g., Lucas and Tanner, 2007); however, Rainforth (2003) has determined that these taxa probably represent tracks made by pseudosuchians.

Zeigler, K.E., and J.W. Geissman. 2008. Magnetostratigraphy of the Upper Triassic Chinle Group and Implications for the Age and Correlation of Upper Triassic Strata in North America. Geological Society of America Abstracts with programs (online).

Mickey commented on my earlier post regarding the recent proclamation of a dinosauromorph specimen from the Induan of Germany. In my original post I stated that crocodile-line archosaurs were present in the Induan in the form of Proterosuchus(see picture below - from Wikipedia). Of course Proterosuchus is a basal archosauriform and NOT a archosaur. I wish thatI could state that this was based on new research that shows that proterosuchids were more derived than previously thought, but of course this is not the case. In fact in an upcoming publication on the enigmatic form Vancleavea (Parker and Barton, in press) we include a phylogenetic analysis that also supports proterosuchids as basal archosauriforms. Thus I really have no excuse except that I simply rushed the original post. Upon reflection, based mainly on Mickey's comment, I realize that there is a little more to say about this find.

The discovery of a dinosauromorph from the Induan (earliest Triassic) would not "fill in" ghost lineages as I said in my earlier post, but rather would extend the ghost lineages for erythrosuchids and euparkeriids back from the Olenekian (late Early Triassic) into the Induan. It would also extend the ghost lineage for proterochampsids and pseudosuchians back from the Ladinian (late Middle Triassic) and Anisian (early Middle Triassic) respectively to the Induan. As a result the ornithodirans would hypothetically appear first in the fossil record in respect for the pseudosuchians. The earliest known described ornithodiran is from the Ladinian.

In my opinion one of the most interesting aspects of the Triassic fossil record is the diversity found in the crocodile-line archosaurs and the bauplan similarities between many pseudosuchians and ornithodirans. In fact I am fond of telling my students that, because the first pseudosuchian appears in the Anisian and the first ornithodiran in the Ladinian, the dinosaurs were simply "crocodile wannabees". I may have to rethink this if this Induan find is confirmed.

REFERENCE

Parker, W.G., and B.J. Barton. In Press. New information on the Upper Triassic archosauriformVancleaveacampi based on new material from the Chinle Formation of Arizona. PalaeontologiaElectronica.

The latest issue of Palaeontology contains two papers on Late Triassic fossils. The first by Stein et al. examines the aerodymanics of the British kuehneosaurs (gliding reptiles - reconstruction below) and contains a brief discussion of their taxonomy. To determine gliding angle and speeds, the team constructed several plastic and aluminum models and set them in a wind tunnel. Results suggest that the elongate "winged" Kuehneosuchus was an effenctive glider and that the short "winged" Kuehneosaurus was a parachutist. Also of interest is discussion that the taxonomic status of these two genera is unclear. One possibility is that they represent sexual dimorphs of the same species. According to this hypothesis the long "winged" individuals would be males given that the more elongate "wings" provide a strong display apparatus. Accordingly the "wings" would have had an original function and were secondary adapted for gliding.

The second paper by Heckert et al. is a description of a new sphenodontid from the Upper Triassic Ghost Ranch Coelophysis Quarry in northcentral New Mexico. Sphenodontid (represented by the extant tuatara - photo to left) fossils from the Chinle Formation are rare and usually incomplete. This specimen is important because of its more complete nature (both dentaries, possible maxilla, and impressions of a portion of the palate, all toothbearing). The new taxon is named Whitakersaurus bermani in honor of the late George Whitaker who originally discovered the quarry in 1947, and Dr. David Berman of the Carnegie Museum who conducted the last major excavations of the quarry in the 1980s. The Ghost Ranch quarry contains an extensive assemblage of Late Triassic vertebrates including 100s of well preserved specimens of the theropod Coelophysis bauri (Colbert, 1989).

Despite the extensive discussion in this paper by Heckert et al. regarding the stratigraphic position of this spectactular fossil assemblage it should be noted that this is still hotly debated and the subject of current research. The quarry is in reddish and green mottled siltstones (Schwartz and Gillette, 1994) which Colbert (1989) assigned to the Petrified Forest Member (Chinle Formation). Interestingly Colbert considered the quarry to be lower stratigraphically than an occurrence of Coelophysis from Petrified Forest National Park that was described by Padian (1986). Contrary to this all other workers have considered the quarry to be high stratigraphically in either the "siltstone" member (e.g., Stewart et al., 1972), the Owl Rock Member (e.g., Dubiel, 1989), or the Rock Point Member (e.g., Hunt and Lucas, 1993) (Column to right from Parker, 2005). Stewart et al. (1972) were unsure of the correlation of the siltstone member with either the Rock Point or the Owl Rock Members. Dubiel (1989) argued that the siltstone member represents a lateral facies of the Owl Rock Member, which is absent in north central New Mexico. Lucas and colleagues have countered this in numerous papers since 1992, arguing that the strata are equivalent to the Rock Point Member both lithologically and bio-stratigraphically. In fact, they have been unwavering in this hypothesis with the section in Heckert et al. (2008) being almost verbatim to that of Hunt and Lucas (1993).

However, this may need to change. A detailed paleomagnetism study of these strata, which is part of a recently completed PhD dissertation by Kate Zeigler, argues that the siltstone member and Rock Point Member possess differing paleomagnetic polarity signals (Zeigler, 2008). Even more striking, they also have different paleo-pole positions. Even more significant is that the siltstone member has a different paleo-pole from the Chinle Formation as a whole and one that is identical to that of the stratigraphically higher Moenave Formation (once believed to be Jurassic, but now probably Upper Triassic). This would not only be a significant deviation from previous hypotheses stratigraphically but also biostratigraphically. The Coelophysis Quarry fauna is dominated by the theropod Coelophysis bauri, but also contains phytosaurs, poposaurs, "rauisuchians", "sphenosuchians", and an enigmatic archosauriform which has tentatively been referred to Vancleavea campi, all of which are common in the stratigraphically lower members of the Chinle Formation. This would significantly extend the stratigraphic ranges of all of these taxa and demonstrate the survival of the majority of pseudosuchian lineages right up to the Triassic/Jurassic boundary. What is needed now is some independent verification of this hypothesis by isotopic dates.

Dubiel, R.F. 1989. Depositional and paleoclimatic setting of the Upper Triassic Chinle Formation, Colorado Plateau; pp. 171-187 in Lucas, S.G., and A.P. Hunt (eds.), Dawn of the Age of Dinosaurs in the American Southwest. New Mexico Museum of Natural History, Albuquerque.

Hunt, A.P., and S.G. Lucas. 1993. Stratigraphy and vertebrate paleontology of the Chinle Group (Upper Triassic), Chama Basin, north-central New Mexico. New Mexico Museum of Natural History and Science Bulletin 2:61-69.

Padian, K. 1986. On the type material of Coelophysis Cope (Saurischia: Theropoda) and a new specimen from the Petrified Forest of Arizona (Late Triassic: Chinle Formation); pp. 45-60 in Padian, K. (ed.), The Beginning of the Age of Dinosaurs. Faunal change across the Triassic-Jurassic Boundary. Cambridge University Press, Cambridge.

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About Me

My name is Bill Parker and I am a paleontologist in northeastern Arizona. While my day job mainly involves roaming the badlands of Petrified Forest National Park picking up bit after bit of phytosaur scrap (and finding some good stuff in-between), thus my main focus is the paleontology of the Triassic Period. However, I also have a very strong interest in Civil War history, and am a direct descendent of veterans who fought on both sides. Note: This is a personal site and all posts are my own opinions and do not represent the opinions of the National Park Service.